1. Field of the Invention
The present invention relates to laminated balun transformers, and more particularly, to a laminated balun transformer preferably for use as a balanced-to-unbalanced converter, a phase transformer, or other suitable component of an integrated circuit in radio communication equipment.
2. Description of the Related Art
A balun transformer is used to convert a balanced signal from a balanced transmission line into an unbalanced signal for an unbalanced transmission line or to convert an unbalanced signal from an unbalanced transmission line into a balanced signal for a balanced transmission line. The term xe2x80x9cbalunxe2x80x9d is an abbreviation of xe2x80x9cBALanced-UNbalancedxe2x80x9d. A balanced transmission line includes a pair of signal lines, through which a signal (balanced signal) is transmitted as a potential difference therebetween. In a balanced transmission line, the two signal lines are equally affected by external noise, thereby canceling out the external noise. Thus, the balanced transmission line is less susceptible to the influence of external noise. In addition, since the internal circuit of an analog IC (integrated circuit) typically includes a differential amplifier, the input/output signal terminals for the analog IC are often balanced terminals that input or output a signal as a potential difference between the two terminals. In contrast, in an unbalanced transmission line, a signal (unbalanced signal) with a potential relative to a ground potential (zero potential) is transmitted through a single signal line. Examples include a coaxial line and a microstrip line on a substrate.
In the past, a laminated balun transformer 1 as shown in FIG. 9 has been provided as a balanced-to-unbalanced converter for a transmission line in a radio frequency circuit. The balun transformer 1 according to the prior art includes dielectric sheets 2a to 2h. On the surface of the dielectric sheet 2b, a lead electrode 3 is provided. On the dielectric sheets 2c, 2d, 2f, and 2g, xc2xc wavelength strip lines 4, 5, 8, and 9 are provided, respectively. On the surfaces of dielectric sheets 2a, 2e, and 2h, shielding ground electrodes 12, 13, and 14 are provided, respectively. The strip lines 4 and 9 are electrically connected in series via a relay terminal N, to define an unbalanced transmission line. The strip lines 5 and 8 define balanced transmission lines, respectively. The strip line 5 is arranged so as to oppose the strip line 4 with the dielectric sheet 2c interposed therebetween. Thus, the strip lines 4 and 5 are electromagnetically coupled (line coupling) to define a first coupler. The strip line 9 is arranged so as to oppose the strip line 8 with the dielectric sheet 2f interposed therebetween. Thus, the strip lines 8 and 9 are electromagnetically coupled (line coupling) to define a second coupler. In FIG. 9, reference numeral 18 indicates a via hole.
In the laminated balun transformer 1 according to the prior art, typically, the lengths of the coupling portion of the strip lines 4 and 5 and the coupling portion of the strip lines 8 and 9 are each set to xcex/4. However, when the length of the coupling portion of the strip lines 4 and 5 is set to xcex/4, it is difficult to set the length of the coupling portion of the strip lines 8 and 9 to xcex/4, due to different lengths of lead portions of the strip lines 4, 5, 8, and 9 and the presence of the relay terminal N. This makes it difficult to adjust the phase of a signal.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a laminated balun transformer that facilitates adjustment of the phase characteristic of a signal.
According to a first preferred embodiment of the present invention, a laminated balun transformer to be mounted on a circuit board having a ground pattern includes a laminate including first and second spiral line elements that define a pair of balanced transmission lines, third and fourth spiral line elements that define an unbalanced transmission line that is electromagnetically coupled with the balanced transmission lines, and a plurality of dielectric layers, wherein the first, second, third, and fourth spiral line elements, and the dielectric layers are laminated together. When the laminated balun transformer is mounted on the circuit board, the distance between the second and fourth line elements, which are electromagnetically coupled, and the ground pattern is greater than the distance between the first and third line elements, which are electromagnetically coupled, and the ground pattern. Where the length of a xcex/4 line element at a given characteristic impedance is L(0), a coupling portion at which the first and third line elements are electromagnetically coupled has a length of L(0)+xcex1 and a coupling portion at which the second and fourth line elements are electromagnetically coupled has a length of L(0)xe2x88x92xcex2, where xcex1 greater than 0 and xcex2 greater than 0.
In general, when the length of a xcex/4 line element at a given characteristic impedance is L(0), arranging the line element into a spiral shape provides substantially the same effect (so-called xe2x80x9creduction effectxe2x80x9d) as using a line element that is shorter than the length L(0). Thus, the coupling portion of the second and third line elements is L(0)xe2x88x92xcex2. As the distance from the two line elements, which are electromagnetically coupled (line coupling), to the ground pattern of the circuit board is increased, a conductor path which electrically connects the two line elements and the ground pattern increases, thereby increasing a parasitic inductance generated in the conductor path. Accordingly, the length of the coupling portion of the first and third line elements is preferably greater than the coupling portion of the second and fourth line elements, that is, to L(0)+xcex1. In this manner, setting the length of the coupling portion of the second and fourth line elements to L(0)xe2x88x92xcex2 and setting the length of the coupling portion of the first and third line elements to L(0)+xcex1 facilitates adjustment of the phase of a balanced signal output from the balanced transmission lines and the phase of an unbalanced signal output from the unbalanced transmission line.
According to a second preferred embodiment of the present invention, a laminated balun transformer having an input impedance lower than the output impedance thereof is provided. The laminated balun transformer includes a laminate including first and second spiral line elements that define a pair of balanced transmission lines, third and fourth spiral line elements that define an unbalanced transmission line that is electromagnetically coupled with the balanced transmission lines, and a plurality of dielectric layers, wherein the first, second, third, and fourth spiral line elements, and the dielectric layers are laminated together. The third line element, which is electromagnetically coupled with the first line element, is connected to an input terminal of the laminated balun transformer, and the second and fourth line elements, which are electromagnetically coupled, are connected to two output terminals, respectively. When the length of a xcex/4 line element at a given characteristic impedance is L(0), a coupling portion at which the first and third line elements are electromagnetically coupled has a length of L(0)+xcex1 and a coupling portion at which the second and fourth line elements are electromagnetically coupled has a length of L(0)xe2x88x92xcex2, where xcex1 greater than 0 and xcex2 greater than 0.
To provide a reduced input impedance, the pattern width of the first and third line elements, which is connected to the input terminal, is increased relative to the second and fourth line elements, which are respectively connected to the output terminals, and/or the thickness of the dielectric layers of a ground electrode and the first and third line elements, which are provided in the balun transformer is decreased. With this arrangement, however, the xe2x80x9creduction effectxe2x80x9d of the first and third line elements is decreased.
Accordingly, the length of the coupling portion of the first and third line elements, which are connected to the unbalanced terminal (input terminal), is preferably greater than the length of the coupling portion of the second and fourth line elements. Thus, according to preferred embodiments of the present invention, the length of the coupling portion of the first and third line elements is preferably L(0)+xcex1, and the length of the coupling portion of the third and fourth line elements is preferably L(0)xe2x88x92xcex2. This arrangement facilitates adjustment of the phase of a balanced signal output from the balanced transmission lines and the phase of an unbalanced signal output from the unbalanced transmission line. In addition, setting the lengths thereof to L(0)+xcex1 and L(0)xe2x88x92xcex2 greatly improves the design versatility without compromising the design and structure, even where it is structurally difficult to design the length of the coupling portion of the first and third line elements to be equal to the length of the coupling portion of the second and fourth line elements.
Preferably, the number of turns of the first and third line elements, which are electromagnetically coupled, is greater than the number of turns of the second and fourth line elements, which are electromagnetically coupled. With this arrangement, without changing the number of turns of the first and third line elements, varying the number of turns of the second and third line elements adjusts the respective inductance components of the second and fourth line elements. As a result, this arrangement facilitates adjustment of the input impedance of the laminated balun transformer. In addition, this arrangement enables fine adjustment of the phase of a balanced signal output from the balanced transmission lines and the phase of an unbalanced signal output from the unbalanced transmission line.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the drawings thereof.